At least two species of human interferon are known to exist which differ in their antigenicity, physicochemical properties and some biological activities. One type is the major component of interferon preparations obtained from short-term cultures of buffy coat cells upon stimulation with parainfluenza virus 2 (Sendai); the other type is either the major or exclusive component of interferon preparations obtained from cultures of human diploid fibroblasts on stimulation with poly(I). poly(C), Sendai or Newcastle disease virus. We plan to first separate and then characterize the distinct human interferon species. The characterization will include: antigenic and physicochemical properties, antiviral activity against selected viruses and antiviral activity in cells of heterologous species. The different species of human interferon will also be compared in their ability to inhibit cell growth in culture (using both diploid and heteroploid cells) and in their inhibitory action on tumor growth in immunologically deficient mice injected with heteroploid human cells. It will be attempted to determine if the different species of human interferon are products of distinct structural genes or whether they arise as a result of modifications of a single gene product. The results of this study are expected to be of importance for the planning of clinical trials with different human interferon preparations, aimed at the evaluation of interferons in the management of viral infections and some forms of cancer.